The present invention relates to a mechanical device for cyclically displaying a series of symbols, one after another, behind an aperture in a cover plate. It relates more particularly to a device comprising a pair of superposed annular disks intended to display information in the aperture of the dial of an analog timepiece, e.g. the date, by means of numerals of substantially larger size than those of conventional single-disk calendars. The invention also relates to a device for quickly correcting the position of the disks.
Calendars that resort to a pair of superposed annular discs to increase the size of the numerals are known. Swiss patent specification 660941, for instance, describes in detail one embodiment of such a calendar in the case of a wristwatch. In this embodiment, the lower disk comprises 17 equal sectors, the first 16 bearing numbers 1 to 16, the last being blank. The upper disk comprises 16 sectors, the first being a window that enables the numerals of the lower disk to become apparent, whereas the following 15 sectors bear numbers 17 to 31. The device further comprises a driving mechanism, within the disks, for rotating the disks in steps of one sector off the hours staff of the watch's movement. At the start, the two disks are in an initial position in which the window and the number 1 are opposite the dial aperture which thus displays the ordinal of the first day of a month. The mechanism first only drives the lower disk through 16 successive steps. Thus, while the upper disk is at rest, the first 15 steps cause numbers 2 to 16 to succeed one another behind the window and the aperture after the number 1 that was visible in the initial position. During the 16th step a shoulder on the lower disk abuts against a similar shoulder on the upper disk to cause the latter to move forward one step. Thus, at the end of the 16th and final step, number 17 of the upper disk that is directly visible and the blank sector of the lower disk that is hidden by the other disk are in register with the aperture. The mechanism then only drives the upper disk through 15 successive steps. Thus, while the lower disk is at rest, the first 14 steps cause numbers 18 to 31 to follow one another behind the aperture. During the 15th step, the above mentioned shoulder on the upper disk abuts against the shoulder of the lower disk to cause the latter to move forward one step. At the end of the 15th and final step the two disks will then be back in their initial position, with the window of the upper disk and the number 1 of the lower disk in register with the aperture. Finally, to avoid accidental movement of the disks as a result of a shock, the device comprises a pair of conventional jumpers, each serving to position one disk.
The cited document also describes a further embodiment in which the upper disk comprises two windows, the lower disk remaining unchanged. With appropriate shoulders, it then becomes possible to display the numbers from 1 to 31 or from 1 to 30, i.e., more generally, both the odd numbers and the even numbers.
The embodiments disclosed in the cited document do however suffer from a number of drawbacks. In particular, the lower disk comprises a blank or void sector, and in the second embodiment the upper disk moreover has a second window. The presence of the void sector and of the second window reduces the space available for the numerals and hence means having to reduce the size of the latter. This runs counter to the object of the invention. Further, because of the central arrangement of the drive mechanism, the disks can be made to have quite a large diameter. But since the moment of inertia of a disk increases to the power four of its radius, the positioning of a large disk requires the jumper to exert much pressure on the disk, hence involving considerable frictional forces. Such frictional forces reduce of course the operational reliability of the watch, particularly at those times when both disks are being driven at the same time.